Since autonomous robots, micro-air vehicles, and prosthetic limbs operate ideally for long periods without refueling or recharging, increasing the gravimetric and volumetric effectiveness of the energy supply and energy conversion equipment is critical. While nature's choice is to chemically power the diverse muscles of her design with a high-energy-density fuel, human kind has largely taken another route. Electrical energy is typically converted to mechanical energy using motors, hydraulic systems, or piezoelectric, electrostrictive, or electrochemical actuators. Because of high electrical power needs, some of the most athletically capable robots are wired to a stationary power source and cannot move freely.
What is needed is an artificial muscle based on a material—an artificial muscle material—that can function as an integral part of a fuel cell, thereby allowing the conversion of chemical energy to mechanical energy in a more direct, compact manner. When speaking about mechanical devices, the terms “activator” and “artificial muscle” are herein defined as devices that can provide a mechanical displacement by using dimensional changes of a solid material or a solid material-liquid transformation.
Chemically powered artificial muscles based on polymer gels were demonstrated over fifty years ago and remain of practical interest for both chemically and electrically powered actuators (H. B. Schreyer et al. in Biomolecules 1, 642-647 (2000) and D. Kaneto, J. P. Gong, and Y. Osada in J. of Mater. Chem. 12, 2169-2177 (2002)). While actuator strain generation can be very large, application has been limited by low response rates, low stress generation capabilities, and the low energy densities of utilized chemical reactions.
Nanoscale and larger actuators have been described that are powered by oxygen gas released by the catalytic decomposition of hydrogen peroxide (W. P. Paxton et al. in J. Am. Chem. Soc. 126, 13424 (2004); S. Fournier-Bidoz, A. C. Arsenault, I. Manners, and G. A. Ozin in Chem. Commun. 2005, 441; T. R. Kline, W. F. Paxton, T. E. Mallouk, and A. Sen in Angew. Chem. Ind. Ed. 44, 744-746 (2005); J. M. Catchmark, S. Subramanian, and A. Sen in Small 1, 202-206 (2005); and R. F. Ismagilov, A. Schwartz, N. Bowden, and G. M. Whitesides in Angew. Chem. Ind. Ed. 41, 652-654 (2002)). Also, gas-powered engines that use the catalytic decomposition of hydrogen peroxide and like materials has been described by R. C. Michelson and S. Reece in U.S. Pat. No. 6,446,909, issued Sep. 10, 2002. Since the catalytic reaction of hydrogen peroxide to produce oxygen is not very energetic, the available energy to power an actuator is not large. None of these actuators has the benefit of using a chemical fuel and oxidant to power a muscle material—the hydrogen peroxide reaction instead involves the formation of water and oxygen. The latter reference (U.S. Pat. No. 6,446,909 uses released gases to power a turbine engine, and this use of a turbine engine increases system weight, volume and cost.
Chemically powered artificial muscles (where non-faradaic charging resulting from redox reactions is used to expand a high surface area material using Coulombic repulsion forces) has been proposed (R. H. Baughman, C. Cui, J. Su, Z. Iqbal and A. A. Zakhidov, U.S. Pat. No. 6,555,945, issued Apr. 29, 2003). However, inventive methods for solving practical problems are still needed and were not provided. Existing problems include:                (a) The described actuators are of the cantilever type and teachings are not provided that enable extension to tensile actuators. Cantilever-based actuators are well known not to be useful for generating mechanical power or large forces, and are therefore very limited in applicability. A problem is that no means were described for mechanically decoupling the actuating electrode from the counter electrode. Consequently, the described chemically powered artificial muscle is limited to a cantilever configuration. Furthermore, the use of a proton exchange membrane and no other auxiliary electrolyte (the proposed preferred implementation therein) inherently prohibits mechanical decoupling of opposite electrodes. Thus, without additional inventive means the membrane and counter electrode parasitically load the actuating electrode and force use as a cantilever device.        (b) The requirement of having an electrolyte that is “substantially gas impermeable” severely restricts the practically usable electrode geometries to those which still permit efficient proton transport. In fact, all substantially gas impermeable electrodes are solids that have much lower ionic conductivities than are obtainable for gas permeable liquid electrolytes. This decreased ionic conductivity for “substantially gas impermeable” ionic conductors means that actuator response rate is lower than would otherwise be possible. Since the ratio of counter electrode surface area to actuating electrode surface area must be large to optimize response rate, the response rate has been limited by the corresponding need for parallel opposite electrodes having the same physical area.        (c) This prior art teaches the need to have separate confinement at opposite electrodes of fuel and oxidizer, thereby requiring separate control of the delivery of fuel and oxidizer. This need for separate confinement of fuel and oxidizer is eliminated in some embodiments of the present invention.        
A fuel-driven shape memory metal based actuator is proposed by R. J. Howard in U.S. Pat. No. 7,135,076, issued Nov. 14, 2006. A fuel/oxidizer (air or oxygen) mixture is applied to a shape memory metal alloy, so that heat released by a catalyzed fuel oxidation can be used to actuate this shape metal alloy (SMA). The dynamic response is limited by the amount of excess air or other dilutent required in the mixture to reduce the reaction temperature to prevent explosion or damage to the shape memory metal as a result of overheating. A large volume of excess air or other dilutent to the fuel concentration simultaneously reduces the heating efficiency by increasing convective heat loss.
The cooling portion of the actuator stroke is also impaired since the mixture flow must either be cut off, leaving the SMA in still air, or the fuel portion of the mixture must be removed, resulting in some latency between command and effect. The cooling rate (and, correspondingly, the actuation rate during cooling) is limited to that achievable by gaseous cooling and conduction from the ends of the SMA. No means enabling this a liquid fuel/oxidizer mixture are proposed, nor are any non-explosive candidate mixtures discussed. Furthermore, inherent to any such mixtures is the added oxidizer weight that severely reduces the specific impulse of the fuel. In an attempt to overcome the gaseous cooling and latency performance limitations during the heating part of the actuation cycle, U.S. Pat. No. 7,135,076 also proposed an additional means of reaction initiation—such as preheating the shape memory material by electrical resistance heating. Such systems and methods are not particularly suited for compact, high cycle, quick response implementation, and likely need an external power source, such as a battery.
Hydrogen-induced actuation of hydrogen absorbing metal alloys has been attempted for micromechanical devices using the known large volume changes induced by hydrogen absorption to make hydrides. However, no reversible actuation was observed (Y. Zhang et al., Proceedings of the SPIE 4601, 131 (2001)). Moreover, the attempted actuation process used hydrogen as a volume-expanding intercalant, rather than as a fuel. The benefits of hydrogen as a high energy density fuel could not have been obtained even if the experiments had been successful.
Thermally powered actuation induced by microwave absorption heating, heating by contact with a thermal reservoir, heating by absorption of light, or resistive electrical heating is well known. In some actuator devices, this heating is used to cause dimensional change of a shape memory composition, such as a shape memory polymer, shape memory polymer composite, a shape memory ceramic, or a shape memory metal alloy. In such devices, heating causes expansion or contraction, and this actuation is reversed on cooling. However, since these devices are not powered by the oxidation of fuel, the benefits or such power source were not obtained.
In addition, cantilever-based thermal actuators based on resistive heating are widely used in Micro-Electro-Mechanical Systems (MEMS). These devices operate somewhat like the cantilever devices in thermostats used for the home—a temperature rise of the cantilever device causes cantilever bending due to differing thermal expansion coefficients of two different materials in the cantilever. This temperature increase results in actuation (cantilever bending) that is electrically driven in the MEMS devices.
Metal alloy and polymer shape memory actuators are types of thermal actuators, and of these, the metal alloy shape memory alloys are widely used and believed to be the most commercially significant. Transition between a low temperature shape memory alloy phase having low elastic modulus (called martensite) and higher temperature phase having high modulus (called austenite) causes the dimensional changes that produce shape memory metal actuation. Widely used shape memory alloys include NiTi, CuZnAl, and CuAlNi alloys.
Shape memory alloys have been previously used with conventional fuel cells to control fuel flow valves, and such control has been achieved using either electrical resistive heating or the waste heat of the fuel cell. These developments are described in United States Patent Application Publication Nos. 20040229094A1 (Bae et al., published Nov. 18, 2004), 20030157385 (Beckmon et al., published Aug. 21, 2003) and 20030162070A1 (Hirsch et al., published Aug. 28, 2003, now U.S. Pat. No. 6,924,055) and Japan Patent JP63016562. However, the shape memory alloy does not provide artificial muscle capabilities for accomplishing mechanical work exterior to the fuel cell and fuel delivery system.
United States Patent Application Publication No. 20040170879A1 (Laurent et al., published Sep. 2, 2004) also uses a shape memory alloy in conjunction with a conventional fuel cell. The shape memory alloy is not used for actuation. Instead the shape memory alloy connects anode and cathode of the fuel cell. The resistance change caused by the phase transition between the lower conductivity martensite phase and the higher conductivity austenite phase is used to control heat-up to the optimal temperature for fuel cell operation.
Japan Patent Application No. JP2001229942 describes use of a shape memory alloy as safety shutoff to prevent the catastrophic destruction of a fuel cell. The SMA valve actuator is activated by fire in the fuel cell assembly, and functions to shut off flow of one of the fuel gases. Actuation is again not accessible for external application, and is not used during normal fuel cell operation.
United States Patent Application Publication Nos. 20050074647A1 (Arthur, published April, 2005) and 20040081866A1 (Bekkedahl et al., published Apr. 29, 2004) describe shape memory springs that are used to move external components into contact with the fuel cell. The purpose is to either short any excess charge on the electrodes prior to start up or after shut down or to provide connection to a heat sink that helps maintain fuel cell temperature. Both patent applications disclose that either excess fuel cell system heat can operate the actuator or an external power source can resistively heat the shape memory springs to provide improved process control. Neither provides actuation that is usable outside the fuel cell system.
The prior art lacks any means for practically converting the energy of a high energy density fuel in an efficient manner to both electrical and mechanical energy. The present invention provides these means for diverse applications. The benefit over electrical actuation is enormous for autonomous systems, since packaged high energy density fuels and delivery systems provide order-of-magnitude or higher advantages in energy storage density compared with the highest performance batteries. This translates to correspondingly increased mission lengths for actuator systems, whether for an autonomous robot or prosthetic limbs.
Additionally, while it is well known and widely utilized that electrical charge injection and electrical heating can change the magnetic, electrical, and optical properties of materials, there is a great need for means for obtaining these changes on command for mobile application where the limited energy storage capabilities of batteries limit mission length.